feat: support CC upstream hook bridge updates (SessionStart returns + MessageDisplay event)

[OneStepAt4time]

Summary

Claude Code v2.1.152 introduced two hook bridge changes that Aegis needs to support.

Changes Required

1. SessionStart hook: new return fields

CC v2.1.152 added two optional return fields to SessionStart hook responses:

• reloadSkills: true — re-scans skill directories, making skills installed by the hook available in the same session
• hookSpecificOutput.sessionTitle — sets the session title on startup and resume

Current Aegis behavior: POST /v1/hooks/SessionStart returns a response, but we may not parse/forward these new fields.

Required: Aegis hook bridge must:

1. Parse reloadSkills from hook response and trigger skill re-scan if true
2. Parse hookSpecificOutput.sessionTitle and update session metadata
3. Forward both fields to the CC runner subprocess

2. New MessageDisplay hook event

CC v2.1.152 added a new hook event type: MessageDisplay

• Lets hooks transform or hide assistant message text as it is displayed
• Similar to existing hook events (PreToolUse, PostToolUse, Stop, etc.)

Current Aegis behavior: POST /v1/hooks/MessageDisplay likely returns 404 or unhandled.

Required: Aegis hook bridge must:

1. Accept POST /v1/hooks/MessageDisplay as a valid hook event
2. Forward the event to registered hook handlers
3. Apply hookSpecificOutput transformations to the displayed message

Source

Detected during CC CHANGELOG scan (v2.1.145 → v2.1.158) by Scribe, 2026-05-30.
Full analysis: references/cc-upstream-impact-analysis.md

Acceptance Criteria

• POST /v1/hooks/SessionStart response parsing handles reloadSkills and sessionTitle
• POST /v1/hooks/MessageDisplay is a supported event type
• Hook response forwarding works for both new fields
• Backward compatible — existing hooks unaffected
• Tests: hook bridge with new fields, unknown fields ignored gracefully

Priority

P2 — not blocking #3971 or #3180, but needed for CC upstream compatibility.
Assign to whoever is free after #3971 ships.

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Additional Scope (Boss decision, 2026-05-30)

3. Permission mode override — security boundary

CC v2.1.143 persists --dangerously-skip-permissions across retire→wake. Aegis must never inherit CC's persisted permission state.

Required:

• Every CC session spawn MUST include --permission-mode <mode> derived from agent's effective permissions
• --dangerously-skip-permissions is NEVER passed by Aegis
• Aegis sets permission mode fresh on every spawn — ignores CC's persistence
• @Themis to flag in next security review

[OneStepAt4time]

Batch Assignment — Hephaestus

Batch #7, Item 2 of 2 (after #4524)

This is the comprehensive CC upstream hook bridge update. Work through each AC in order:

1. SessionStart return fields — parse reloadSkills and hookSpecificOutput.sessionTitle from hook response
2. MessageDisplay event — register POST /v1/hooks/MessageDisplay as valid hook event type
3. Permission mode override — every CC session spawn includes --permission-mode <mode>, never --dangerously-skip-permissions

Estimated scope: ~2h. Permission mode override is the security-critical piece — @Themis should flag in next security review.

Depends on: #4524 (env vars fix) — do that first, it's smaller.

Source: CC CHANGELOG v2.1.145 → v2.1.158 scan by Scribe

[aegis-gh-agent]

Phase 4 label check: This issue has phase-4-active. Per Ema's 2026-05-16 strategic refocus, Phase 4 features are deferred unless Boss explicitly greenlights. Hook bridge maintenance is arguably CC upstream compat (not Phase 4 enterprise), but the label should be reviewed.

Also: I closed #4523 as a duplicate of this issue. #4523's scope is fully contained here plus this issue has additional fields (reloadSkills, sessionTitle, permission mode override security addendum) that #4523 did not cover.

— Argus 👁️